Best Speaker Placement Rules you've used?

Diagonally across a room corner, so the central listening position is located at 1/3 room length and width.
ie: for 12x15 room, listener is 4' from one side wall and 5' from the back wall.

I've done some basic frequency response measurements with various arrangements, and this one produces by far the smoothest and most extended bass response.
It also tends to reduce the impact of early sidewall reflections, since the sidewalls "splay outward" like a horn, instead of being perpendicular to the speaker baffles.

From a speaker manufacturer now out of business, I learned to measure the center of the woofer to the floor and multiply that time 1.6. This is the distance to the side or back wall. This then is multiplied by 1.6 and that is the distance to the other. This is NOT likely to be pleasing to others in your life.

I have used the directions on the Cardas and Audio Physic websites with excellent results. AP has changed their directions, and they aren't quite as well written as in the past.